AT&T MERLIN LEGEND System Reference Manual page 511

Hardware
Line/Trunk and Station Modules
Figure 5-4 Processor Module
A NiCad battery in the processor module provides backup power for the real-
time clock and nonvolatile RAM in case of power failure or system shutdown.
The battery provides RAM data retention for 12 to 30 days. The trickle-charge
circuit can recharge the battery to 50 percent of capacity from a discharged
state in 48 hours. The minimum battery life is five years.
The line/trunk and station modules have jacks for connecting the telephone
company lines/trunks and the station lines to the control unit. The station lines
connect to telephones and adjuncts such as answering machines and fax
machines.
Depending on the mode of operation chosen, the outside lines that extend from
the telephone company's central office are called trunks or lines. "Trunks" is
used for Hybrid/PBX mode and in the phrase "tie trunks." "Lines" is used for Key
or Behind Switch modes.
Control Unit 5-5